The invention relates generally to an improved thermodynamic cycle in which a heat-exchange medium goes through a cyclic process of compression, condensation, and expansion to produce power or to produce cooling. More specifically, the invention relates to an improved thermodynamic cycle that uses hydrostatic head to effect compression.
Thermodynamic power-producing cycles and refrigeration cycles use various cyclic combinations of compression, heat rejection, expansion, and heat addition. In these cycles, the cost of energy to effect compression is a significant part of the cycles"" capital cost as well as the cycles"" operating cost.
Common features of thermodynamic power-producing cycles include (1) a compression process wherein one or more compressors or pumps increase the pressure of a heat-exchange medium, (2) a thermal addition process in which energy is added to he heat-exchange medium from a suitable heat source such as air, water, or fossil fuel, 3) an expansion process during which work is done by the system on the surroundings, and (4) a rejection process in which thermal energy is transferred to the surroundings. In the process, thermal energy is converted into mechanical energy.
Refrigeration is used in the oil and gas industry to recover heavier hydrocarbons from produced fluids. This is desirable for various reasons, including hydrocarbon dewpoint conditioning and recovery of natural gas liquids (NGLs) that are more valuable as a liquid than as a gas. Refrigeration is also used in the production of liquefied natural gas (LNG). In the production of offshore oil and gas, the refrigeration is often on offshore structure where space is a premium. The refrigeration system typically requires equipment to compress, condense, and expand a heat-exchange medium to produce cooling. Having a compressor on an offshore structure takes up space and increases safety risks associated with having a mechanical apparatus using a pressurized heat-exchange medium which is often combustible. It would be desirable therefore to have a refrigeration system available for use in offshore operations without having a compressor on the surface facilities.
It would be beneficial from the standpoint of both initial and operating costs to eliminate the need for a conventional compressor in power-producing cycles and refrigeration cycles.
The present invention is direct to a thermodynamic cycle that uses compression and expansion to generate refrigeration or power in which a substantial portion of the compression is effected by hydrostatic head of the heat-exchange medium used in the cycle.
In one embodiment, a refrigeration cycle uses the head of a heat-exchange medium in the refrigeration cycle to compress the heat-exchange medium. Vaporous heat-exchange medium is introduced into the upper end of a down riser or conduit that extends downwardly through a suitable heat sink. The vaporous heat-exchange medium descends through the down riser and the head of the heat-exchange medium compresses the heat-exchange medium. The heat generated by the compression of the heat-exchange medium in the down riser is transferred to the heat sink, which surrounds the down riser. Preferably, the heat-exchange medium leaves the bottom of the down riser in a liquid phase. The heat-exchange medium liquid is then pumped up through a return riser and passed through a pressure expansion means and evaporator. From the evaporator the heat-exchange medium is returned to the upper end of the down riser for recycling.
In this embodiment of the present invention eliminates the need for the mechanical compressor of a conventional refrigeration system. Compression and heat rejection phases of this refrigeration system are simultaneously performed in a down riser. A heat-exchange medium pump is the only moving part of the refrigeration system. Compression of the heat-exchange medium is virtually isothermal at the temperature of the heat sink.
In another embodiment of this invention, a thermodynamic power cycle uses hydrostatic compression to condense a heat-exchange medium. In this embodiment, a heat-exchange medium is compressed by downward flow in a gravitational field from an initial elevation to a second, lower elevation. Examples of suitable systems for achieving the elevation difference can comprise a wellbore or a conduit extending between the top and bottom of a body of water. At the lower elevation, the heat-exchange medium is pumped back to the initial, upper elevation. The heat-exchange medium is then heated, thereby at least partially vaporizing the heat-exchange medium. The heated heat-exchange medium is then passed through an expansion device, preferably one or more hydraulic turbines, to expand the heat-exchange medium to a lower pressure whereby energy is produced. From the expansion device the heat-exchange medium is preferably cooled by a heat exchanger before being recycled.
The present invention can substantially reduce the cost of compressing the gas to a liquid compared to the cost of an adiabatic process performed by a conventional compressor. One advantage of this invention over power cycles used in the past is that gravity is used to carry out a substantial portion of the compression of the heat-exchange medium.